Exercise therapy in healthy sedentary adults improves limb strength by about 30%, increases aerobic capacity (VO2max) by about 15%, reduces depression and anxiety, and increases mood and self esteem. An increasing body of evidence indicates comparable benefit occurs in disease states that are relevant to ALS, such as post-polio syndrome - a disease of the lower motor neuron for which increased strength and aerobic capacity has been demonstrated - and multiple sclerosis - a disease of the upper motor neuron for which improved strength, aerobic capacity, mood, and quality of life has been shown. Neither the efficacy nor the safety of exercise therapy for ALS, or any of the other neurogenic diseases represented by the MDA, is known. The specific aims of this study are to determine if a supervised 4-month exercise training program can safely improve muscle strength, aerobic capacity, pulmonary function, mental health, and qualify of life in patients with ALS. The design is parallel, randomized, controlled, and blinded, and the sample size is 40 patients. If exercise increases muscle strength by the same mean magnitude in ALS as for healthy sedentary adults, it would represent a gain comparable to the expected mean decline in muscle strength over 9 months of the disease course. The possibility also exists that in ALS psychosocial benefits may be found from exercise which will surpass the physiological ones (strength and aerobic capacity). The recent data from multiple sclerosis (MS) is encouraging wherein exercise therapy had a significant impact in improving quality of life, strength, and endurance. Exercise therapy is an inexpensive, universally available, therapeutic modality. Improvement by exercise therapy of any of these measures without adverse effect would be expected to have a significant impact on clinical practice, not only for ALS, but also for other neurogenic diseases causing weakness.